1. Field of the Invention
The present invention relates in general to speed sensors. Particularly, the present invention relates to speed sensors used for measuring the travel speed of a motor vehicle. More specifically, the present invention is concerned with rotation speed sensors of a type which uses a magnetic detecting means.
2. Description of the Prior Art
In order to clarify the task of the present invention, one conventional rotation speed sensor of the above-mentioned type will be described with reference to FIG. 4 of the accompanying drawings, which is used for measuring the travel speed of a motor vehicle.
In FIG. 4, there is shown the conventional sensor which is generally designated by numeral 100. Denoted by numeral 102 is a pinion gear which is meshed with a gear (not shown) mounted on a drive shaft of an automotive transmission. The pinion gear 102 is fixed to one end of a rotation shaft 104 which is rotatably held in a plastic housing 106. The rotation shaft 104 has at the other end an annular supporter 108 on which an annular magnet unit 110 is concentrically mounted. The magnet unit 110 consists of a plurality of magnet elements which are arranged about the supporter 108 in such a manner that N-pole and S-pole appear alternately. Near one side of the annular magnet unit 110, there is placed a Hall-effect sensor element 112. Wires extend from the Hall-effect sensor element 112 to an electronic processing circuit which is printed on a print board 114. Denoted by numeral 116 is a socket member which has connector pins 118 connected through wires, (not shown) to terminal ends of the electronic processing circuit on the print board 114. A spring washer 119 is compressed between the annular supporter 108 and the housing 106 to bias the annular magnet unit 110 toward the Hall-effect sensor element 112.
However, due to its inherent construction, the above-mentioned conventional rotation speed sensor 100 has the following drawbacks.
First, because the smooth and stable rotation of the rotation shaft 104 depends chiefly on the physical intimacy between the shaft 104 and the housing 106, it is necessary to finish both the rotation shaft 104 and the housing 106 (viz., bore for the shaft 104) accurately with a highly skilled machining technique. In fact, in this conventional sensor 100, the rotation shaft 104 tends to receive a complex radial load from the wall of the bore of the housing 106.
Second, for increasing the wear and abrasion resistance of the rotation shaft 104, it is necessary to harden the outer surface of the same by using a surface hardening technique.
Third, because the rotation shaft 104 is designed to hold the pinion gear 102 and the magnet unit supporter 108, a milling process at "a" and a knurling process at "b" are necessary after machining the shaft 104.
These matters, however, bring about an increase in production cost and a troublesome production process.